Recently, as a next-generation NVRAM (Nonvolatile Random Access Memory) capable of operating at high speed to replace a flash memory, various kinds of device structures such as a FeRAM (Ferroelectric RAM), an MRAM (Magnetic RAM), an OUM (Ovonic Unified Memory) are proposed and fierce development race is carried on to implement high performance, high reliability, low cost, and process consistency. However, current each above memory device has good and bad points and it is far from an ideal “universal memory” having good points of an SRAM, a DRAM and a flash memory.
Based on the existing technique, a method of changing an electric resistance reversibly by applying a voltage pulse to a perovskite material known for its colossal magnetoresistance effect is disclosed in the following patent document 1 and non-patent document 1 by Shangquing Liu and Alex Ignatiev et al. in U.S. Houston University. This is an extremely epoch-making method in which while the perovskite material known for its colossal magnetoresistance effect is used, a resistance change over several digits can be provided at room temperature without applying a magnetic field. An RRAM (Resistance Random Access Memory) including a variable resistance element using this phenomenon has superior characteristics such that a power consumption is extremely low because a magnetic field is not needed at all unlike the MRAM, miniaturization and high integration can be easily implemented, and a multilevel storage can be implemented because a dynamic range of the resistance change is considerably larger than the MRAM.
As shown in FIGS. 1 and 2, the basic structure of the actual variable resistance element is extremely simple such that a lower electrode 3 serving as a second electrode, a variable resistor 2 and an upper electrode 1 serving as a first electrode are sequentially laminated. Here, FIG. 1 is a perspective view showing a basic structure of the variable resistance element, and FIG. 2 is a sectional perspective view taken along a line A-A in FIG. 1. In addition, according to an element structure exemplified in the patent document 1, a lower electrode 3 composed of a yttrium barium copper oxide YBa2Cu3O7 (YBCO) film is deposited on a single-crystal substrate (not shown) of lanthanum aluminum oxide LaAlO3 (LAO), and a variable resistor 2 sandwiched by the lower electrode 3 and an upper electrode 1 is a crystalline praseodymium calcium manganese oxide Pr1-XCaxMnO3 (PCMO) film which is a perovskite-type oxide and the upper electrode 1 is composed of an Ag film deposited by sputtering. As an operation of the variable resistance element, it is reported that the resistance can be reversibly changed by applying positive and negative voltage pulses of 51V between the upper electrode 1 and the lower electrode 3. It is said that a new nonvolatile memory device can be implemented by reading a resistance value in the reversible resistance changing operation (hereinafter referred to as the “switching operation” occasionally).
In addition, a ZnSe-Ge hetero structure or a metal oxide such as Ti, Nb, Hf, Zr, Ta, Ni, V, Zn, Sn, In, Th or Al is known as the material of the variable resistor 2 other than the perovskite material whose resistance value can be changed according to application condition of a voltage pulse although it is small.
Patent document 1:U.S. Pat. No. 6,204,139
Non-patent document 1: Liu, S.Q. et al., “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films”, Applied Physics Letter, Vol. 76, pp. 2749-2751, 2000